In certain techniques for targeted blood-brain barrier opening, including using focused ultrasound (FUS), it can be desirable to increase targeting accuracy while decreasing the time and effort necessary for accurate targeting. Systems and techniques for blood-brain barrier opening using FUS are described in U.S. Patent Application Publication No. 2009/0005711, which is incorporated by reference herein in its entirety.
Targeting accuracy can be reduced by aberrations of the ultrasound beam caused by the skull. The discrepancy between the high speed of sound through the skull and the low speed through the underlying brain tissue, alone or along with attenuation of ultrasound waves through the skull bone, can distort the beam shape, including at higher frequencies. Moreover, the trabecular layer of the skull can induce heterogeneities in both speed of sound and density, which can lead to phase aberrations of the acoustic beam. At higher frequencies, the defocusing effect of the skull can be increased as the wavelength can reach the size of local skull bone heterogeneities (for example, the trabeculae can be around 1 mm). The phase aberrations can be reduced by reducing the ultrasound frequency. However, the size of the focal region can likewise increase, which can increase the likelihood of undesirable inertial cavitation.
In therapeutic ultrasound, it can also be desirable to have real-time monitoring and treatment efficiency verification. A passive cavitation detector (“PCD”) can be used to transcranially acquire the acoustic emissions stemming from the microbubble. The frequency analysis of backscattered signals can be relevant to characterize undesirable bubble-capillary interaction.